To skip or not to skip

If the entry plan required the spacecraft to make a skip, perhaps to extend the flight path as happened on Apollo 11, the computer advanced to P65 which controlled the ascending part of the skip-out trajectory. Whether the skip caused the spacecraft to briefly leave the atmosphere depended on how far the flight path was being extended from entry interface. Nevertheless, there was a program, P66, ready to revert attitude control to the RCS thrusters should P65 sense that deceleration had dropped sufficiently low. If the spacecraft did not rise out of the atmosphere, as was the case with Apollo 1 l’s skip, then P66 was not used and P65 handed over to P67. If there was a second re-entry, it would be P66 that passed control to P67. In most cases however, re-entry did not include a skip-out phase so P64 handed directly to P67.

In the case of Apollo 11. as they approached Earth on their three-day coast, the weather in the prime recovery area looked increasingly poor so the decision was taken to maintain their trajectory and revise the re-entry flight path to include a skip – out. thereby extending their flight through the atmosphere from 2,200 kilometres to nearly 2,800 kilometres. “1 wasn’t very happy with that,” said Collins at his debrief, "because the great majority of our practice and simulator work had been done on a 1.187 [2.200-km] target point. The few Limes we fooled around with long-range targets, the computer’s performance and the ground’s parameters seemed to be in disagreement. So. when they said 1,500 miles [2.800 kilometres], both Neil and I thought. ‘Oh God. we’re going to end up having a big argument about whether the computer is Go or No-Go for a 1,500-mile entry.’ Plus 1,500 miles is not nearly as compatible. It doesn’t look quite the same on the EMS trace. If you had to take over, you’d be hard-pressed to come anywhere near the ship. For these reasons. I wasn’t too happy about going 1.500 miles, but I cannot quarrel with the decision. The system is built that way and. if the weather is bad in the recovery area. 1 think it’s probably advantageous to go 1.500 miles than to come down through a thunder­storm.”